This invention relates generally to adaptive equalizers and specifically to techniques for determining the operating mode of an adaptive equalizer. The prior art discloses two general types of automatic equalization. The first type is referred to as preset equalization in which a transmitted training signal or sequence is compared at the receiver with a locally generated training sequence, with the differences between the compared training sequences being used to set the coefficients of the equalizer. In the second, adaptive equalization method, the coefficients of the equalizer are continually and automatically adjusted based upon the received data. It is also known to use preset equalization initially to provide good channel error performance, and once normal transmission begins, to switch to an adaptive algorithm for the equalizer in which equalization is determined by the data.
Both of the foregoing equalization techniques may further be controlled by adjusting the step size (sometimes referred to as the gain factor) used to update the tap coefficients of the equalizer. A large step size will cause the equalizer to converge quickly while rendering it more sensitive to noise and other channel impairments. A smaller step size will stabilize the equalizer but reduce its operating speed. The term "operating mode" is used herein and in the claims in a broad sense to denote any method of controlling the operation of an equalizer, including selectively operating the equalizer in response to a received training signal (sequence) or received data or controlling the step size of the equalizer in either of the foregoing modes.
In the preferred embodiment of the invention, a received digital signal, which may comprise a television signal, a data signal or any other information bearing signal, is constituted by a plurality of multilevel symbols, including a fixed pseudo random sequence of symbols used as a field synchronizing signal. The received digital signal also includes a pilot comprising a DC offset characterizing the symbols. In one aspect of the invention, the field sync signal is used to determine the DC variations in the received signal, which variations are indicative of the rate of change of a received ghost component. For fixed or very slowly moving ghosts, the equalizer is operated in response to the field sync signal, which is used as a training sequence. In the presence of more rapidly moving ghosts, however, field sync directed equalization is not fast enough and a data directed operating mode for the equalizer is used. The invention is concerned with the method of determining when to switch between the field sync directed equalization mode and the data directed equalization mode.
In another aspect of the invention, a portion of the field sync is subjected to successive field delays and subtracted from the corresponding portion of the current field sync to develop values indicative of changes in the field sync over successive fields. A comparator determines when the changes occurring in successive fields are sufficiently different from a predetermined threshold level for the adaptive equalizer to be switched into its data directed mode from its field sync directed mode.
In a further aspect of the invention, the equalizer is operated in response to the field sync signal and its step size is controlled by the DC variations in the received signal or the changes in the field sync signal over successive fields. In particular, the step size of the equalizer is increased when a rapidly moving ghost is indicated and decreased when a slowly moving or stationary ghost is indicated.